What is a SIP trunk? A SIP trunk is a call that is routed by IP over the Internet through an Internet Telephony Service Provider (ITSP).
For enterprises wanting to make full use of their installed IP PBXs and communicate over IP not only within the enterprise, but also outside the enterprise, a SIP trunk provided by an ITSP that connects to the traditional PSTN network is the solution. Unlike traditional telephony, where bundles of physical wires were once delivered from the service provider to a business, a SIP trunk allows a company to replace traditional fixed PSTN lines with PSTN connectivity via a SIP trunking service provider on the Internet.
SIP trunks can offer significant cost savings for enterprises, eliminating the need for local PSTN gateways, costly ISDN BRIs or PRIs. The following figure is an example of how our phone system operates:

You can see that we have a local area network containing our desktops, servers, phones, and our 3CX Phone System. To reach the outside...